RAT build debug

[Jay]

Hi,

I've built a RAT on a Tonepad board and can't get it to work - no output.

I can follow audio into the board, goes into the LM308 on Pin 3 then stops.

Voltages as follows:

1 8.37
2 8.07
3 7.50
4 0.00
5 0.00
6 8.04
7 8.86
8 7.28

So most of them are sitting too high, pin 2 should be about half rail, 3 a bit less than that etc.

If I remove the chip I get the rail on pin 7 and 3 sits about half of that.  Which is what you'd expect.

I've tried a different op-amp, I've confirmed correct components in the correct place, I've looked and looked again for shorted tracks but no joy.

No doubt a large Homer style Doh! on the way when I eventually get it but I can't see for looking.

Anyone got any clues please?


thanks,


Jay.

[PerroGrande]

Check the distortion pot (100k-log) to make sure it is properly wired and that the pot is good.  Check the solder connections on the 4.7 and 2.2 uF caps near the distortion pedal connection.

If you built this circuit with a commercial board, the biggest chance for problems remains the soldering (cold joints, bridges, lifted traces, etc).


(No insult intended by the following questions.  I need to ask the basic questions.)

Did you verify the correct orientation of the op-amp?
When you tried a different op amp, what type was it? 
Are you using an LM308 or something different?

[Jay]

Hi,

Thanks very much for your for the suggestions.

I did have the distortion pot wired wrong, but corrected it and still no go (my voltage measurements are with it corrected).  If I measure with the chip removed I see 100K in the feedback path now.

I'm using an LM308 - I didn't have another one to try so I used an LF351.  I assumed, maybe wrongly that any single op-amp would work, just maybe not sound right.

I've been over all the soldering, and I've looked for solder bridges.

I've tried removing, measuring the volts, then restoring the following:

The compensation cap, the feedback cap, one end of the 560R, one end of the 47R.  I was hoping one of these would be the cause.

The little thing is beating me!

What makes the DC conditions of an op-amp do that?


thanks,


Jay.

[Jay]

Problem solved.

Dead LM308.

I subbed in a TL071 and it roared, almost literally, into life.

Maybe it was dead from the word go, or maybe I had something wrong which also killed the LF351.

Either way it works now!

Jay.

[PerroGrande]

Not too surprising, actually.   Short of some odd solder bridging and the like, when an op-amp behaves like that, it is often toasted.   Glad to hear you've got it working!